Convex hyperboloid mirrors are useful in many optical applications. These optical applications often require precision-ground mirrors in optical instruments. The slightest aberration on the reflective surface of the mirror may hinder the performance of the instrument or render the instrument useless. Thus, it is critical that the surface of a convex hyperboloid mirror be tested to ensure that optical surface quality or optical figure across the mirror surface is within close tolerances.
One apparatus used to test the optical surface quality of a mirror surface includes a hindle sphere reflector. In the classical hindle test of a convex hyperboloid, a wavefront of light passes through an aperture at the center of the hindle sphere to test the mirror surface. This aperture creates a "blind spot" in the test pattern corresponding to a central region of the mirror. In addition, the diameter of the hindle sphere must be substantially larger than that of the hyperboloid being tested.
An alternative to the hindle sphere test is the hindle shell test. A hindle shell is a lens having a concave and a convex surface. The convex surface is only used in transmission, but the concave surface is used in transmission and reflection. Because the hindle shell is used in transmission, material optical homogeneity must be very good. Both surfaces must also be optically polished. In addition, the diameter of the hindle shell must be greater than the diameter of the hyperboloid.